The present invention relates to a method of producing a coated substrate having barrier properties.
Substrates having barrier properties are of great importance for packaging food, beverage, or other products that are sensitive to environmental influences. Those substrates generally are provided with a barrier layer using well-known coating techniques such as blade coating, bar (rod) coating, reverse roll (film) coating, or air knife coating. However, each of these application methods has its own set of problems that can result in inferior barrier quality. Furthermore, a common feature of all these methods is that the amount of coating liquid applied to a paper web, which generally has an irregular surface with hills and valleys, is different depending on whether it is applied to a hill or a valley. Therefore, the coating thickness and thus the barrier properties will vary across the surface of the coated substrate resulting in barrier irregularities. Moreover, said methods are also limited in how thin a coating layer may be applied to the substrate. Another drawback of said coating methods known in the prior art that at each coating station only a single layer can be applied to the substrate. If several barrier layers are to be applied to a substrate, then each of said coating layers needs a separate coating station or subsequent coating in a further coating machine. This sequential approach to making multilayer coatings is undesirable in that the subsequent coating steps of the prior art fail when attempting to apply an additional layer to a layer that is very hydrophobic and water repellent. Despite their drawbacks, these coating methods are still the dominant processes in the paper industry due to their economics, especially since very high line speeds can be achieved.
Curtain coating is a relatively new coating technique. EP-A 517 223: and Japanese patent applications JP-94-89437, JP-93-311931, JP-93-177816, JP-93-131718, JP-92-298683, JP-92-51933, JP-91-298229, JP-90-217327, and JP-8-310110 disclose the use of curtain coating methods to apply one or more pigmented coating layers to a moving paper surface.
More specifically, the prior art relates to:                (i) The curtain coating method being used to apply a single layer of pigmented coating to a basepaper substrate to produce a single-layer-pigmented coating on paper.        (ii) The curtain coating method being used to apply a single priming layer of pigmented coating to a basepaper substrate prior to the application of a single layer of pigmented topcoat applied by a blade type coating process. Thus a multilayer-pigmented coating of paper was achieved by sequential applications of pigmented coating.        (iii) The curtain coating method being used to apply a single topcoating layer of pigmented coating to a basepaper substrate that has previously been primed with a single layer of pigmented precoat that was applied by a blade or a metering roll type coating process. Thus a multilayer-pigmented paper coating was achieved by sequential applications of pigmented coating.        (iv) The curtain coating method being used to apply two single layers of specialized pigmented coating to a basepaper substrate such that the single layers were applied in consecutive processes. Thus a multilayer-pigmented coating of paper was achieved by sequential applications of pigmented coating.        
The use of a curtain coating method to apply a single layer of pigmented coating to the surface of a moving web of paper, as disclosed in the prior art discussed above, is stated to offer the opportunity to produce a superior quality coated paper surface compared to that produced by conventional coating means. However, the sequential application of single layers of pigmented coating using curtain coating techniques is constrained by the dynamics of the curtain coating process. Specifically, lightweight coating applications can only be made at coating speeds below those currently employed by conventional coating processes because at high coating speeds the curtain becomes unstable and this results in an inferior coated surface. Therefore, the conventional methods of producing multi-coated papers and paperboards employ the blade, rod or roll metering processes. Unfortunately, the. application of consecutive single layers of pigmented coatings to paper or paperboard at successive coating stations, whether by any of the above coating methods, remains a capital-intensive process due to the number of coating stations required, the amount of ancillary hardware required, for example, drive units, dryers, etc., and the space that is required to house the machinery.
The curtain coating method for the simultaneous coating of multiple layers is well known and is described in U.S. Pat. Nos. 3,508,947 and 3,632,374 for applying photographic compositions to paper and plastic web. However, photographic solutions or emulsions have a low viscosity and a low solids content, and are applied at low coating speeds.
In addition to photographic applications, the simultaneous application of multiple coatings by curtain coating methods is known from the art of making pressure sensitive copying paper. For example, U.S. Pat. No. 4,230,743 discloses in one embodiment the simultaneous application of a base coating comprising microcapsules as main component and a second layer comprising a color developer as a main component onto a travelling web. However, it is reported that the resulting paper has the same characteristics as the paper made by sequential application of the layers. Moreover, the coating composition containing the color developer is described as having a viscosity between 10 and 20 cps at 22° C.
JP-A-10-328613 discloses the simultaneous application of two coating layers onto a paper web by curtain coating to make an inkjet paper. The coating compositions applied according to the teaching of that reference are aqueous solutions with an extremely low solids content of 8% by weight. Furthermore a thickener is added in order to obtain non-Newtonian behavior of the coating solutions. The examples in JP-A-10-328613 reveal that acceptable coating quality is only achieved at line speeds below 400 m/min.
In view of the deficiencies of the prior art, it would be desirable to have an economical, improved process for preparing substrates, such as paper or paperboard, having barrier properties.